1. Field of the Invention
This invention relates to a hydraulic pressure control apparatus for use in an automatic transmission for an automobile.
2. Description of the Prior Art
In a hydraulic pressure control apparatus for use in an automatic transmission, a speed stage is determined in relation to the throttle position in an intake path and vehicle speed, and usually the shift-down is carried out from a third speed to a first speed through a second speed. Since the speed stage in the second speed and the third speed is performed usually by the engagement of a hydraulic frictional engaging device, the output torque of an engine is decreased when a throttle valve in the intake path is almost fully closed, and the drive torque becomes negative when the speed change is carried out from the third speed to the second one, thereby presenting a problem in that blowing sounds take place in a drive system.
The hydraulic pressure control apparatus proposed to overcome such a problem comprises:
a throttle valve for producing a throttle pressure as a hydraulic pressure related to an intake throttle position;
a governor valve for producing a governor pressure as a hydraulic pressure related to the vehicle speed;
a 1-2 shift valve for changing over the first speed and the second speed in relation to the throttle pressure and the governor pressure; and
a 2-3 shift valve for changing over the second speed and the third speed in relation to the throttle pressure and the governor pressure and permitting a line pressure from the 1-2 shift valve in the second speed position to act as the third speed position holding pressure so as to reduce a working force due to the throttle pressure in the third speed position of a D-range.
Accordingly, since in the low throttle position and the 2-3 shift valve the working force due to the throttle pressure is offset by the third speed position holding pressure, the 2-3 shift valve serves to hold the third speed position until the governor pressure reaches a very small value, i.e., the vehicle speed reaches a very small value. On the other hand, the 1-2 shift valve is changed over from the second speed position to the first speed position by the throttle pressure when the governor pressure is reduced and thereby the third speed position holding pressure also disappears in the 2-3 shift valve so that the 2-3 shift valve is changed over from the third speed position to the second speed position. Consequently, the change-over of the 2-3 shift valve from the third speed position to the second speed position is carried out after the second speed position is changed over to the first speed position in the 1-2 shift valve. Therefore, the shift-down from the third speed to the first speed is carried out without shifting to the second speed.
Further in the 2(second) range, when the vehicle speed drops to a predetermined value, the 2-3 shift valve is changed over from the third speed position to the second speed position and thereafter the 2-3 shift valve has to be held in the second speed position irrespective of the vehicle speed. For that end, the 2-3 shift valve in the 2-range receives the hydraulic pressure for the 2-range in the direction opposed to the working force due to the governor pressure.
On the other hand, since the line pressure is used for the engaging pressure of the frictional engaging device, it is set as a function of the intake throttle position, i.e., the output torque of the engine. However, as above mentioned, when the line pressure is utilized for the third speed position holding pressure in the 2-3 shift valve, this holding pressure varies with the intake throttle position, therefore the vehicle speed in the change-over of the 2-3 shift valve from the third speed position to the second speed position in the 2-range does not reach a predetermined value but unfavorably varies.
Also in the hydraulic pressure control apparatus for use in the automatic transmission for controllably fixing the vehicle speed to the second speed in the 2(second) range, the engaging pressure needs to be introduced in the frictional engaging device for the second speed irrespective of the position of the 1-2 shift valve in the 2-range. Thus, the second speed holding valve is provided in the 2-range which introduces the line pressure as the engaging pressure to the frictional engaging device for the second speed irresepctive of operation of the 1-2 shift valve. However, if this second speed holding valve sticks to a position in which the line pressure is introduced to the frictional engaging device for the second speed, the frictional engaging device for L(low) range and the frictional engaging device for the second speed are put into the engaging condition simultaneously in the L-range, thereby causing damages.
Further, when the intake throttle position is almost nil and the up-shift is carried out, if the frictional engaging device for low speed stage and for high speed stage are both of hydraulic pressure system, the release of said device for low speed stage is delayed and the engagement of said device for high speed stage is quickened by the low output torque of the engine. As a result, an overlapping period during which both frictional engaging devices are under the engaged condition may be elongated to produce the negative torque in the frictional engaging device for low speed stage, causing sometimes shocks and uncomfortable blow sounds.